Production of ductile high strength galvanized steel

ABSTRACT

ADVANCING A STRIP OF HOT ROLLED LOW CARBON STEEL, IN THE ABSENCE OF AN APPLIED INTERMEDIATE OXIDIZING TREATMENT, THROUGH A CONTINUOUS ANNEALING FURNACE AS IT IS RECEIVED FROM A COLD REDUCTION MILL WITH A FILM OF ROLLING OIL, HAVING PREFERRED CHARACTERISTICS, ON ITS SURFACE; HEATING THE STRIP TO AN ANNEALING TEMPERATURE IN THE RANGE OF 9001100*F. STRIP TEMPERATURE (1100-1300*F. FURNACE TEMPERATURE) IN A PROTECTTIVE ATMOSPHERE WHICH IS PREFERABLY OF HIGH HYDROGEN; SUBSEQUENTLY, COATING THE ANNEALED STRIP WITH MOLTEN METAL, PREFERABLY GALVANIZING MATERIAL.

23, 1974 J. E. I-IARTMANN E ,4 8

PRODUCTION OF DUCTILE HIGH STRENGTH GALVANIZED STEEL Filed Oct. 26, 1972 PRIOR ART HOT cow REDUCE -0XlDlZ-lNG 7 ZONE BAND NON-FATTY F'OE ANNEAL AsH- FREE H2 ATMOSPHERE OIL IGNITION SOURCE N2 PURGE SYSTEM FIG. I

COATING METAL I I I HARDNESS RB I I I I l I I00 H RDNESS A R so FIG.3

United States Patent 3,825,448 PRODUCTION OF DUCTILE HIGH STRENGTH GALVANIZED STEEL John E. Hartmann, Canfield, and Alfred F. La Camera and Peter B. Lake, Youngstown, Ohio, assignors to Youngstown Sheet and Tube Company, Youngstown,

Ohio

Filed Oct. 26, 1972, Ser. No. 301,126 Int. Cl. 'C23c 1/02, 1/04, 1/08 US. Cl. 148-12.1 7 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION Cold rolled steel strip, as it is received from a rolling mill, has rolling oil on it which is deposited during the rolling operation. To prepare the cold rolled steel strip and render it suitable for coating with other metal, e.g., galvanize, it has been considered necesary to remove or convert the oil residue in order to obtain good adherence of the metal coating.

Under some prior art practices, the oil is removed by special liquid or electrolytic cleaning procedures. In prior art methods which require a precleaning step, additional equipment and special handling is required. Special handling tends to reduce production rates. Also, in such methods which include pickling, difficulties have been encountered with unsatisfactory and irregular coating surfaces and poor adherence of the coating to the strip.

In those processes which include the purposeful formation of an oxide film, and the subsequent reduction of the oxide to effect the removal thereof or reconverting the oxide film into the base metal, the temperatures employed for reduction or conversion of the oxide film are such as to fully anneal the product. While a fully annealed product displays an enhanced ductility characteristic there is an attendant reduction in yield strength.

The present invention provides a process which obviates the need of special precleaning after cold reduction and prior to annealing. The invention further provides a process capable of producing a high strength (at least 80,000 p.s.i. yield strength) and adequate ductility for roll forming applications.

A desideratum for ductile high strength galvanized steel is a capability for producing on available existing facilities. However, the problems associated with such procedures deter such approach. One of the better known lines for processing galvanized steel is that designated in the trade as the Sendzimir process, such as disclosed in US. Pats. 2,110,893 and 2,197,622. There is disclosed a line whereby strip cleaning is conducted by first oxidizing the strip in a preheat or oxidizing furnace and subsequently re ducing the formed oxide in an annealing furnace which is operated at a temperature in the range of 1500-1800 F.

OBJECTS AND SUMMARY OF THE INVENTION It is an object of this invention to provide a ductile high strength metal coated steel, and process for producing the same, which satisfies the above mentioned desideratums. It is a further object of this invention to provide a process for producing a ductile high strength galvanized steel, which process may be conducted without the need for special cleaning procedures, particularly those requiring the use of baths, chemical or electrolytic, or for the purposeful formation of oxides.

Briefly, the objects are attained by cold-reducing steel strip, of the designated compositions, in the presence of an ash-free non-fatty rolling oil; heat treating to partially anneal the cold-reduced strip, in the absence of a purposeful intermediate oxidizing treatment, to a temperature in excess of 900 F., but not exceeding 1100" F., preferably in the range of 10501080 F., in an atmosphere of dissociated ammonia and for a time to vaporize the oil and to produce a steel which is not fully recrystallized; and applying a molten coating to the heat treated strip.

The desideratum of avoiding the purposeful formation of oxides by reducing the temperature in or shutting off the oxidizing furnace, creates other concomitant problems, e.g., burn-off of the rolling oil is not possible with the limited supply of oxygen available in the hydrogen atmosphere annnealing furnace. Generally, it would be expected that a potential explosion hazard would occur through the use of dissociated ammonia to create the hydrogen atmosphere. It might also be expected that adherence of the coating to the strip would be alfected because of the contemplated reduced operating temperatures in the annealing zones.

Other details, objects and advantages of the invention will become apparent from the following description of a present preferred method of practicing the same.

DESCRIPTION OF THE DRAWINGS DETAIL DESCRIPTION OF THE INVENTION The following examples disclose the ranges of the principal elements of the embodiments of compositions from which high strength ductile galvanized steel, within the scope of the present invention may be prepared:

Percent by weight Preferred Element Acceptable Carbon (C) 07-. 20 10-. 13 Manganese (Mn) 20-. 60 .35. 60 Phosphorus (P). .010 max.

the balance being iron with residual impurities and an ingredient or ingredients not subversive of the high strength and ductile characteristics of the steel.

In the process of this invention, an annealing strip temperature of about 1050-1100 F. was contemplated. In the accepted standard Sendzimir practice, previously referred to, the strip is heated in an oxidizing atmosphere in a preheater (phantom lines, FIG. 1) to a ternperature of approximately to 500 C. (338932 F.) to burn the oil and produce a surface oxidation. The surface oxide is subsequently reduced in a reducing furnace, usually operated at an average standard tern perature of 1700 F. Information in the literature suggests that a decrease in the rate of iron oxide reduction of approximately tenfold could be expected by the contemplated reduction in furnace temperature. Since burnoff of the rolling oil is not possible without the operation of the oxidizing furnace, other techniques for the removal of the oil and/ or for deterring oil residue accumulation in the furnace had to be developed. It was decided to adopt a process using a rolling oil, during the cold reduction, of ash-free quality and having a flash-point below the temperature of the first zone in the annealing furnace. These characteristics enable the residual rolling oil to be vaporized with a minmum of residue contaminants.

In an embodiment of this invention, a low carbon steel is first hot rolled at a finishing roll temperature generally in the range of 1560l720 F., preferably 1570- 1630" F., and at a coiling temperature below 1200" F., preferably in the general range of 10801120 F., to form a hot band. In a preferred embodiment the band is formed to a 0.080 inch maximum thickness. It is important, in order to promote good coating adherence, that the cold reduction (40 to 90%) be conducted with the application of a non-fatty and ash-free rolling oil and that a good shape (high degree of flatness) be produced. A typical procedure calls for a .080 inch hot band to be cold reduced to .020 inch on a four-stand or a five-stand mill. An oil which is suitable for this purpose is presently sold by the Quaker Chemical Corporation as a proprietary item under the trade name designation Quakerol 7096-75. The available literature indicates that this oil is a complex mixture of natural occurring, highly polar, high and low titer synthetic lubricants, couplers, bactericides, specifically refined hydrocarbons and nonionic emulsifiers. The oil is preferably used as a 2% solution with water and applied at all of the stands of the rolling mill. A detergent solution may optionally be applied at the last stand.

The cold reduced strip is next heat treated without any intermediate cleaning or purposely applied oxidizing treatment. It is believed that a thin film of the rolling oil deters oxidation of the strip surface. In the case where there is an oxidizing furnace in existing annealing lines, such as in Sendzimir lines, the oxidizing furnace is turned off or down so that the temperature therein is below 500 F., and hence will not function in its oxidizing capacity.

The cold reduced strip is continuously heat treated in an annealing furnace, having a temperature in the range of 1l00l300 F. and a protective atmosphere to provide a strip temperature of 9001100 F., preferably 1050l080 F. While the annealing temperature will generally be in the indicated range, it varies depending upon the composition of the steel. The time of annealing is limited so that the steel will not be recrystallized at all, or if it is, it will not be fully recrystallized but will contain small finite grains in a striated matrix. The microstructure of the steel will be substantially homogeneous throughout its thickness and correspond somewhat to that described in US. Pat. 3,264,144 to Frazier et al., e.g., the recrystallization will be in the range of -95%.

Without exposing the heat-treated steel to the external atmosphere, the steel strip is lead to a molten bath of the coating metal to be applied, which coating metal in a preferred embodiment of this invention is a galvanize composition. It was unexpectedly found that the reduced operating temperatures do not adversely affect adherence of the coating to the strip and that a galvanize product having high strength and increased ductility is produced.

Four steels of the following compositions were used to develop the hardness curves and corresponding annealing response parameters:

Constituents, percent by weight The balance of each composition being essentially iron. However, the compositions may include an ingredient or ingredients not subversive of the high strength and ductile characteristics of the steel.

FIG. 2 shows the hardness curves 1, 2, 3, and 4 developed for the correspondingly numbered four steel compositions. The annealing response parameter M is a time-temperature one in the basic form or. T

dt t(2.303)

and is derived from a rate equation for diffusion; see Larson and Salmas, A Time-Temperature Relation for Recrystallation and Grain Growth, Transactions of the American Society for Metals, vol. 46, p. 1377. The letter C is a constant and for steel material of the type used has been established to have a value of 20. A preferred method of this invention embodies annealing temperatures and times which produce an annealing response parameter M falling (substantially within a hardness band formed between the curves 2 and 4.

In FIG. 3 are curves showing the relation between the hardness, as measured on a Rockwell B scale, and the strengths of the steel compositions 2, 3 and 4. Thus, it will be observed that a yield strength of about k.s.i. corresponds to a Rockwell B hardness value of about 85. Therefore, in order to produce a steel having a yield strength of at least 80 k.s.i., the steel will have a hardness of at least 85. Correspondingly, it will be observed that the processing which will produce such steel requires an annealing response parameter M of about 1.65 to 1.88. It is preferable to select an annealing parameter M which does not correspond to a point on the portion of the hardness curve having a steep slope. Since the slope of the band of curves becomes steeper as M is increased above approximately 1.88, it is preferred to stay within the range of 1.65 to 1.88. As the slope of the hardness curve increases, it becomes correspondingly more difficult to maintain accurate control of the temperature and time which will produce a steel having hardness value falling on the curve.

The curves shown in FIG. 2 were derived from tests conducted to establish the annealing behavior of the 'various metallurgical compositions. The results of these tests also indicate that the recovery region is extended as the carbon content is increased. The recovery region may be considered to be that portion of the annealing cycle where relief of the stresses is initiated and recrystallization has not commenced. An extended recovery region is considered to be advantageous in the control of inline continuous annealing to produce steels at the higher strength levels. Therefore, compositions having the higher carbon contents are preferred.

In a preferred annealing practice of this invention, an annealing atmosphere comprising about 75% hydrogen is used. In any high hydrogen 10%) furnace atmosphere a potential explosion hazard exists it operated at low temperatures and with the presence of oxygen. To deter the possibility of an explosion occurring, the aim operating temperatures in the annealing furnaces were restricted to minimums of 1100 F. Additionally, an ignition source was provided to deter any build-up of hazardous quantities of oxygen and hydrogen. The specific ignition source provided is in the form of a radiant tube heater, one in each of the seven zones of the furnace used. A manual control for the ignition source is pro vided for full fire operation whenever low-temperature operation of the annealing temperature source is required. A purging system, which introduces nitrogen into the furnace system in the event of an electrical power failure is also provided.

What is claimed is:

1. Process for treating steel to produce a high strength annealed and metal coated steel product characterized by good ductility, which process comprises:

(a) cold reducing a hot-rolled steel strip to which a non-fatty rolling oil has been applied;

(b) annealing said strip, in the absence of an applied intermediate cleaning treatment, at a temperature and for a time less than that which will produce a fully recrystallized steel but suflicient to vaporize the residue of the rolling oil; and

(c) advancing said strip in a protective atmosphere to and for a metal coating application.

2. A process as described in Claim 1, wherein: the steel being annealed comprises, in percent by weight, .07- .20 carbon, .20-.6O manganese, and With the balance being essentially iron with residual impurities and an ingredient or ingredients not subversive of the high strength and ductile characteristics.

3. A process as described in Claim 2, wherein: said strip is annealed in a furnace having a temperature in the range of 1100l300 F.

4. A process, as described in Claim 1, for treating steel strip in a furnace having a high hydrogen atmosphere, which includes: providing an ignition source to ignite quantities of hydrogen which might otherwise be explosion hazardous.

5. A process as described in Claim 1 to produce a product characterized by a tensile elongation characteristic of at least 3 percent, which process further comprises:

providing a hot rolled low carbon steel which has been finish rolled at a temperature below 1720 F., and coiled at a temperature below 1200 F.;

cold reducing such hot rolled steel 40 to 90%; and

6 annealing the cold reduced steel, at a temperature in the general range of 1050 to 1080 F. arid for a time to vaporize the non-fatty oil and to produce a steel of substantially homogenous structure.

6. A process, as described in Claim 1, of treating steel strip in a furnace, which comprises: advancing the strip through the furnace for a time and temperature which is related to the equation where M=an annealing response parameter, which includes time and temperature as a single function, T=temperatnre in K.

r=time in seconds where the strip temperature is 900-1100 F., and M is in the order of 1.65 10 to 1.88 10 7. A process, as described in Claim 1, wherein: said strip is annealed in the absence of an applied oxidizing treatment.

References Cited UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION July 23, 1974 Patent N0 8 2 5 Dated John E. Hartmann et a1. Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Claim 6, -----I .T dt t'(2.303) Should read dt 0 Signed'ahd sealed this 5th day of November 1974.

' (SEAL) Attest:

McCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents "ORM PO-IOSO HO'SQ) USCOMM.DC 60375 269 u.s. covnmnsm rnm'rmc omcz: 930 

